The invention relates to collet arrangements.
A common type of collet arrangement uses the so-called drawback collet. This collet has collet segments which together define an aperture for receiving the work piece and provide a tapered external surface. The collet is threadedly attached to a draw bar or draw tube which draws the collet axially and rearwardly into a closing taper, that is, a tapered tube in which the collet is a sliding fit. Engagement between the outer tapered surfaces of the collet segments and the corresponding inner taper of the tube results in inward radial movement of the collet segments so as to grip the workpiece.
Although the drawback type of collet provides good accuracy consistent with axial concentricity, it has certain disadvantages resulting from the movement of the collet which is necessary to grip the workpiece. For example, this type of collet is not easy to use where a second operation has to be carried out on a workpiece, because it is difficult to achieve the required accuracy in the length of the workpiece and in its axial displacement relative to the front face of the collet. The drawback type of collet is also difficult to use where partially finished components have to be picked off from a main spindle to a sub-spindle for subsequent machining; it is difficult to obtain the necessary strict control of axial position of such partially finished components, especially when they are extremely short. It is also difficult or impossible to use a fixed back stop with this type of collet.